The present invention relates to multi-well plates and column arrays in which samples are analyzed or processed.
In recent years, microtitration wells have assumed an important role in many biological and biochemical applications, such as sample preparation, genome sequencing, and drug discovery programs. A variety of multi-well arrangements, constructed according to standardized formats, are now popular. For example, a tray or plate having ninety-six depressions or cylindrical wells is arranged in a 12xc3x978 regular rectangular array is one particularly popular arrangement.
In some multi-well constructions, a filter sheet or membrane is held against the lower ends, or lips, of open-bottomed wells. Such plates are often manufactured as a multi-layered structure including a unitary sheet of filter material disposed to cover the bottom apertures of all the wells, the filtration sheet being sealed to the outer lip of one or more of the well apertures. The use of a single sheet of filter material in such a manner, however, can lead to cross-contamination between adjacent wells due to the ability of liquid to disperse, e.g., by wicking, across the sheet.
In an effort to overcome this problem, it has been proposed to provide each well with its own discrete filter element or disc. According to one such design, a pre-cut filter disc is inserted into an upper, open end of each well and pushed down until it rests at the bottom of the well. An O-ring is then press-fit down into each well until it comes to rest against the top of the filter disc. The O-ring frictionally engages the column inner wall, thereby retaining the filter in place. While avoiding the cross-contamination problems of unitary filter sheets, such a construction is obviously cumbersome to manufacture. Also, the portion of the disk that gets pinched between the O-ring and the floor of the well introduces a significant xe2x80x9cdead volume,xe2x80x9d which can have an adverse impact on sample purification. For example, sample matrix can become entrapped in these areas along a significant portion of the peripheral edge of individual filter discs. When purifying DNA from blood samples, entrapment of small amounts of hemoglobin (heme) on the edges of a cellulose blot membrane will eventually contaminate the final product in the last stages of the purification process. The contaminating heme residue is a strong inhibitor in PCR and sequencing reaction assays of the DNA products.
Another multi-well arrangement, wherein each well has its own discrete filter element, is formed by positioning a single sheet of filter material between an upper plate, having a plurality of mini-columns formed therein, and a lower plate having a plurality of corresponding xe2x80x9cdrip directors.xe2x80x9d Upon bringing the plates together and forming an ultrasonic bond therebetween, the filter sheet is die-cut into individual filter discs positioned below respective mini-columns. Although this construction is easier to manufacture than the above arrangement, it suffers similar disadvantages. Specifically, a substantial portion of each filter disc""s peripheral edge becomes pinched between the column plate and the drip director plate, resulting in a significant dead volume that can adversely impact sample purification.
There is, thus, a need for a multi-well microfiltration arrangement that is relatively simple to manufacture, and that overcomes the problems associated with the prior arrangements relating to cross-contamination due to wicking across a common filter sheet, or individual filter discs entrapping sample constituents within substantial dead volumes.
Most of the known multi-well filtration plates, and particularly those providing an individual filter disc for each well, lack adequate space below the filter element to permit an evenly distributed flow of fluid across the filter. In many arrangements, a drip director, at the bottom of each well, provides an expansive, flat surface upon which much of the filter element rests. Preferential flow pathways are thereby created, favoring those areas of the filter element that are not in contact with, or in close proximity to, the drip director surface. Such preferential flow can have an adverse impact on the elution of solutes. For example, preferential flow pathways can impede the leaching of retained sample constituents in non-favored regions of the filter element.
On the other hand, a lack of adequate support beneath each filter element can be problematic, as well. The filter media used in multi-well trays are typically quite thin and exhibit relatively poor mechanical properties. In certain stressful situations, e.g., high-pressure or vacuum filtration, such membranes may not maintain their integrity. Filter discs that are supported only about their peripheral edges might sag, particularly along their central regions, and may even pull loose from the structure holding their edges. For example, a filter disc might collapse into the cavity of a drip director. This would affect the porosity of the filter, trapping certain sample constituents in the filter that would otherwise elute. Moreover, if a bypass forms along the edges of the filter, due to the filter disc pulling away from the peripheral supporting structure, an undesirable loss of sample may result.
There is, thus, a need for a multi-well microfiltration arrangement that adequately supports the filter media at each well, without creating substantial preferential flow.
A few of the known multi-well microfiltration arrangements provide a collection plate, for placement beneath a sample-well plate, having a plurality of closed-bottom collection wells corresponding to the sample wells. Generally, the collection of filtrate takes place upon application of a vacuum to pull the mobile phase through each well. With most of these arrangements, attempts to separately collect the filtrate from each sample well have suffered from unreliable results due to cross-contamination between the wells of the collection plate. A principal cause of such cross-contamination relates to the production of aerosols as the filtrate leaves the drip directors. The aerosols can readily disperse and travel to neighboring collection wells. In addition, aerosols may expose technicians to potentially pathogenic microorganisms, and the like, which may be present in the samples.
Cross-contamination due to aerosol formation is exacerbated by the typical flow pattern induced by the vacuum arrangements of such systems. Usually, the sample-well plate is mounted above the collection plate, and the collection plate, in turn, sits in a vacuum chamber. Upon evacuation of the chamber, solution within each well is drawn down through the filter element toward a respective collection well. Generally, the vacuum draws along flow pathways extending from within each mini-column, through a respective drip director, and horizontally across the top of the collection plate until reaching one side of the collection plate whereat the flow pathways turn downward toward an exit port. Except for those drip directors located directly adjacent the side of the chamber having the exit port, substances (e.g., entrained aerosols, gases, etc.) pulled along each vacuum flow pathway from each drip director must pass by neighboring collection wells as they travel across the top of the collection plate. Unfortunately, aerosols from filtrate exiting one drip director can become entrained in the flow across the collection plate and make its way over into neighboring wells.
The potential for cross-contamination is particularly high when the upper sample-well and drip-director plates are removed from the collection plate. Pendent drops of filtrate remaining on the drip directors can inadvertently fall into neighboring wells as the drip directors are moved over the collection plate. With standard multi-well plates, a concerted, manual xe2x80x9ctouch-offxe2x80x9d of all such pendent drops to the inner sides of respective collection wells is difficult, if not impossible, due to the great number of wells. Application of a strong vacuum below the drip directors, in an attempt to pull such pendent drops down and away from the drip directors, can atomize the pendent drops, resulting in the related problem of contamination by aerosol formation, discussed above.
There is, thus, a need for a multi-well microfiltration arrangement that provides for the separate collection of filtrate from each well, while avoiding cross-contamination due to aerosol formation and/or pendent drops.
One aspect of the present invention provides a microfiltration apparatus for processing a plurality of fluid samples.
According to one embodiment, the microfiltration apparatus of the invention includes a first plate having a plurality of columns and a second plate having a plurality of discharge conduits. Each of the columns has a first inner bore defining a lumen within the column and an end region for receiving a filter medium within the column. The column end region defines a second inner bore having a diameter greater than that of the first inner bore and a transition region that joins the second inner bore to the first inner bore. A filter medium for filtering sample is positioned within each column end region, adjacent the transition region. Each discharge conduit has an upstanding upper end region aligned with and received within a corresponding column end region so as to form a substantially fluid-tight interface therebetween. The discharge conduit upper end region has a terminal rim region for supporting a circumferential region of the filter medium such that each filter medium is held between a column transition region and the terminal rim region of a corresponding discharge conduit.
In one embodiment, the transition region of each column has an annular tapered portion. The circumference of the annular tapered portion decreases in a substantially constant fashion along a direction from the second inner bore to the first inner bore. In a related embodiment, a line running along the tapered portion, longitudinally with respect to the column, forms an acute angle with a plane perpendicular to a longitudinal axis of the column and intersecting the column through a junction of the transition region with the second inner bore. The acute angle, in one embodiment, is within the range of about 30-70 degrees. Preferably, the acute angle is within the range of about 30-60 degrees. In one particular embodiment, the acute angle is about 45 degrees.
According to one embodiment, the terminal rim region of each discharge conduit contacts no more than about 15%, and preferably less than about 10%, and more preferably less than about 5% of the bottom surface area of a respective filter medium.
One embodiment provides a plurality of fin-like support buttresses in each of the discharge conduits. In this embodiment, each of the support buttresses has an elongated., narrow, uppermost surface that is substantially coplanar with a plane defined by the terminal rim region of a respective discharge conduit. In a related embodiment, the horizontal cross-sectional area of an upper region of each support buttress decreases in a direction extending towards its uppermost surface in a fashion such that the intersection of the uppermost surface with the plane of the terminal rim region is substantially tangential in nature, forming a line.
According to another embodiment, the microfiltration apparatus is provided with a gas-permeable matrix comprised at least in part of a porous hydrophilic polymer material. The matrix is attached to the second plate on a face opposite the first plate. Also in this embodiment, the matrix circumscribes a plurality of the discharge conduits.
A further embodiment provides means for shifting the first and second plates in either of two directions from a reference xe2x80x9chomexe2x80x9d position along a generally horizontally extending axis, and then returning the plates back to the reference xe2x80x9chomexe2x80x9d position. The shifting means can include a stepper motor disposed in mechanical communication with the plates such that angular rotation of the stepper motor induces linear motion of the plates.
In accordance with another embodiment, vacuum means are provided for drawing adherent drops of fluid hanging from the discharge conduits in a direction away from the collection wells and up into the discharge conduits.
In another of its aspects, the present invention provides a method for forming a plurality of microfiltration wells. In one embodiment, a sheet of filter medium is positioned between a first plate containing a plurality of columns and a second plate having a plurality of discharge conduits. Each of the columns has a first inner bore defining a lumen within the column and an end region defining a second inner bore having a diameter greater than that of the first inner bore and a transition region that joins the second inner bore to the first inner bore. Each of the discharge conduits has an upstanding upper end region facing the first plate and aligned with a corresponding column end region. The plates are pressed together in a manner effective to punch portions of the filter medium from the sheet to afford a filter medium plug situated within the end region of each column in abutment with the column transition region and a terminal rim region of a corresponding discharge conduit upper end region.
The method of the invention also provides for the compression-fit sealing of each filter element In one embodiment, compression of each filter element between the column transition region and a terminal rim region of a corresponding discharge conduit upper end region serves to secure and seal the filter element to an inner sidewall of the column.
In another embodiment, the method further includes the step of securing the first plate to the second plate. The securing step can be effected by forming a bond, such as an ultrasonic weld, between an inner sidewall of each second inner bore and an outer circumferential surface of a respective upper end region.
A further aspect of the present invention provides a microfiltration apparatus for processing a plurality of fluid samples.
In one embodiment, the apparatus includes a first plate having a plurality of columns. Each of the columns contains, at one end thereof, a filter element and a fluid discharge conduit beneath the filter element. A second plate is spaced apart from the first plate by a cavity. The second plate has a plurality of receiving or collection wells that are aligned with the columns for receiving sample fluid from the discharge conduits. The second plate is also provided with a plurality of vents adjacent the collection wells. A gas-permeable matrix is positioned in the cavity between the first plate and the second plate so as to fill the space between the confronting surfaces of the two plates. The matrix laterally surrounds the region between at least one discharge conduit and an aligned collection well. The matrix is effective (i) to permit a vacuum drawn from beneath the second plate to extend, via the vents, to a region above the second plate and to the columns, thereby drawing fluid from the columns into the collection wells and (ii) to obstruct movement of aerosols across the top of the second plate, thereby limiting cross-contamination between wells.
According to one embodiment, the matrix is a continuous sheet having a plurality of openings permitting the passage of filtrate from each discharge conduit to a respective collection well. Each one of the discharge conduits can extend at least partially into a respective one of the openings. Further, the matrix can extend over a plurality of the vents. In one embodiment, the matrix is comprised of a porous hydrophilic polymer material, such as ethyl vinyl acetate (EVA) or the like.
In one embodiment, the collection wells are arranged in a rectangular array having at least eight wells (e.g., 8, 12, 24, 48, or 384 wells). In one preferred arrangement, the second plate is provided with at least one vent for every four collection wells, and the vents are arranged such that a vent is located between each collection well and at least one adjacent collection well. For example, a vent may be provided between each collection well and at least one diagonally adjacent collection well of the array.
According to one embodiment, each of the columns has a first inner bore defining a lumen within the column and an end region defining a second inner bore, having a diameter greater than that of the first inner bore, and a transition region that joins the second inner bore to the first inner bore. Each of the discharge conduits has an upstanding upper end region aligned with and received by a corresponding column end region so as to form a substantially fluid-tight interface therebetween. The discharge conduit upper end region has a terminal rim region for supporting a circumferential region of the filter element such that each filter element is held between a column transition region and the terminal rim region of a corresponding discharge conduit.
In another embodiment, means are provided for shifting the first plate in either of two directions from a reference xe2x80x9chomexe2x80x9d position along a generally horizontally extending axis, and then returning the plate back to the reference xe2x80x9chomexe2x80x9d position. The shifting means can include a stepper motor disposed in mechanical communication with the plate such that angular rotation of the stepper motor induces linear movement of the plate.
In a further embodiment, vacuum means are provided for drawing adherent drops of fluid hanging from the discharge conduits in a direction away from the collection wells and up into the discharge conduits.
Another aspect of the present invention provides a method for separately collecting filtrate from an array of microfiltration wells in a corresponding array of closed-bottom collection wells held by a collection tray situated below the microfiltration-well array.
In one embodiment, the method includes the steps of:
(A) placing a fluid sample in a plurality of the microfiltration wells;
(B) drawing a vacuum along pathways extending from each microfiltration well downward through a plane defined by an upper surface of the collection tray at a point at or adjacent a corresponding collection well to a region beneath the collection tray, thereby causing a filtrate to flow from each microfiltration well and to collect in corresponding collection wells; and
(C) obstructing aerosols formed from the filtrate at any one microfiltration well from moving across the upper surface of the collection tray to a non-corresponding collection well, thereby limiting cross-contamination.
According to one embodiment, each vacuum pathway passes through a gas-permeable matrix disposed in a cavity between the microfiltration-well array and the collection-well array. The gas-permeable matrix can be comprised of a porous hydrophilic polymer material, such as ethyl vinyl acetate (EVA) or the like. In one preferred arrangement, the gas-permeable matrix circumscribes the region between each microfiltration well and a corresponding collection well.
In one embodiment, the vacuum pathways pass through the plane of the collection-tray upper surface by way of vents that traverse the collection tray proximate each of said collection wells. Also in this embodiment, the gas-permeable matrix covers the vents.
In another embodiment, each of the vacuum pathways extends from one microfiltration well into a respective collection well prior to passing through the vents.
In a further embodiment, wherein a collection tray having open-bottom wells is used, the vacuum pathways pass through the plane of the collection-tray upper surface and then down and out of the open bottoms of the wells.
The microfiltration wells comprise, according to one embodiment, a first plate having a plurality of columns and a second plate having a plurality of discharge conduits. Each column has a first inner bore defining a lumen within the column and an end region for receiving a filter medium within the column. The end region defines a second inner bore having a diameter greater than that of the first inner bore and a transition region that joins the second inner bore to the first inner bore. A filter medium for filtering sample is positioned within each column end region, adjacent the transition region. Each discharge conduit has an upstanding upper end region aligned with and received within a corresponding column end region so as to form a substantially fluid-tight interface therebetween. The discharge conduit upper end region has a terminal rim region for supporting a circumferential region of the filter medium such that each filter medium is held between a column transition region and the terminal rim region of a corresponding discharge conduit.
In one embodiment, the method includes the additional steps of:
(i) touching-off, in a substantially simultaneous fashion, adherent drops of fluid hanging from the bottom of each microfiltration well to an inner sidewall of a respective collection well; and
(ii) drawing adherent drops of fluid hanging from the discharge conduits in a direction away from the corresponding collection wells and up into the discharge conduits.
In another of its aspects, the present invention provides an apparatus for avoiding cross-contamination due to pendent drops of fluid hanging from a plurality of discharge conduits disposed in an array above a corresponding array of collection wells.
According to one embodiment, the apparatus includes:
(i) a carriage configured to carry one of the arrays and adapted for linear reciprocal motion in either of two directions along a first, generally horizontal, axis from a neutral position whereat the arrays are substantially axially aligned;
(ii) a stepper motor;
(iii) a linkage assembly mechanically communicating the stepper motor with the carriage such that each rotational step of the stepper motor induces movement of the carriage a given distance from the neutral position in one of the two directions depending upon the direction of angular rotation of the motor, thereby effecting relative motion between the discharge-conduit array and the collection-well array such that pendent drops of fluid hanging from the discharge conduits are simultaneously touched-off to inner sidewalls of corresponding collection wells; and
(iv) a compression spring mounted within the linkage assembly in a manner permitting the spring (a) to provide a predetermined amount of resistance to movement of the carriage from the neutral position, and (b) to compensate or absorb some of the linear overshoot due to excess angular rotation of the motor beyond the amount required to move the discharge conduits into firm abutment with the inner sidewalls of the collection wells.
In one embodiment, a vacuum chamber communicates with the discharge-conduit array from a side thereof opposite the collection-well array. Evacuation of the vacuum chamber is effective to urge pendent drops of fluid hanging from the discharge conduits in a direction away from the collection wells and into the discharge conduits.
In one preferred embodiment, the carriage is configured to carry the discharge-conduit array, while the collection-well array remains stationary. A vertical positioning assembly can be disposed on the carriage to support the discharge-conduit array for linear movement along a second, generally vertical, axis between a lowered position whereat the discharge conduits extend down into respective collection wells and an elevated position whereat the discharge conduits clear the collection wells.
Still a further aspect of the present invention provides a method for avoiding cross-contamination due to pendent drops of fluid hanging from a plurality of discharge conduits disposed in an array above a corresponding array of closed-bottom collection wells.
In one embodiment, the method includes the steps of
(i) touching off, in a substantially simultaneous fashion, pendent drops of fluid hanging from the discharge conduits to inner sidewalls of respective collection wells; and
(ii) drawing pendent drops of fluid hanging from the discharge conduits in a direction away from the corresponding collection-well array and into the discharge conduits.
The touching-off step can be carried out by shifting the discharge-conduit array along a plane substantially orthogonal to the longitudinal axes of the collection wells, while the collection wells are maintained in a substantially fixed position. In one embodiment, each of the discharge conduits is shifted into contact with one sidewall portion of a respective collection well, and then is shifted into contact with another, laterally opposing sidewall portion of the respective collection well.
One embodiment provides a stepper motor in mechanical communication with the discharge-conduit array such that angular rotation of the stepper motor induces linear motion of the discharge conduits. In this embodiment, stepping of the stepper motor causes the discharge-conduit array to shift.
The step of drawing pendent drops of fluid can be effected by establishing a reduced pressure (a vacuum) above the discharge conduits.
In one embodiment, an upstanding upper end region of each of the discharge conduits is received within a respective column, thereby forming an array of microfiltration wells. Each column has a first inner bore defining a lumen within the column and an end region defining a second inner bore having a diameter greater than that of the first inner bore and a transition region that joins the second inner bore to the first inner bore. A filter element is disposed in each column, between the transition region of the column and the upper end region of a respective discharge conduit.
In another of its aspects, the present invention provides a removable cover for isolating a plurality of samples separately contained in an array of closed-bottom wells supported in a collection tray.
According to one embodiment, the cover includes a substantially rigid, rectangular shell portion having a top surface, a bottom surface and a circumferential side-edge region. A plurality of reversibly expandable, tubular sleeves are provided on the top surface of the shell portion. A resiliently compliant undersurface is secured to the bottom surface of the shell portion. A plurality of resiliently deflectable, elongated side arms project below the bottom surface from opposing side-edge regions of the shell portion. In its normal (unstressed) state, each side arm is positioned substantially perpendicular to a plane defined by the bottom surface. An inwardly directed catch is formed at an end of each side arm, distal from the shell portion. The arms, and associated catches, are useful for releasably snap-locking the cover over the wells of a collection tray.
In one embodiment, the undersurface of the cover includes a plurality of downwardly convex nodules (half-dome features) disposed in an array complementary to the collection-well array. Each nodule is adapted to fit over a corresponding well when the cover is secured over the collection tray.
A further aspect of the invention provides a method for covering an array of open-top wells held in a collection tray.
According to one embodiment, the method is carried out in a substantially automated fashion using (i) a support structure adapted for movement along a generally horizontal plane (x/y direction) and (ii) a plurality of elongated, parallel rods depending from the support and adapted for movement along their respective longitudinal axes (y direction). Initially, the rods, while disposed in a retracted position adjacent the support, are positioned over a cover member. Two of the rods are then extended away from the support (y direction) so that their end regions become wedged in respective cavities formed along the top of the cover, while two rods are maintained in the retracted position (i.e., with free end regions). The cover member is then lifted by retracting the wedged rods back toward the support. The support is then moved along the x/y direction so that the cover becomes positioned over the collection tray. The wedged rods are then extended away from the support so that the cover is lowered onto the collection tray, over the well openings. The free ends of two retracted rods are then extended until they abut an upper region of the cover, thereby blocking upward movement of the cover, while the wedged rods are retracted away from the cover so that they are withdrawn (unwedged) from the cavities. As a result, the cover is left resting on top of the collection tray over the well openings.
From this position, the cover member can be releasably snap-locked to the collection tray. This can be effected, for example, by extending at least one of the rods away from the support and into abutment with an upper region of the cover, thereby pressing the cover into locking engagement with the collection tray. Another of the rods can be extended away from the support and into abutment with another upper region of the cover in order to prevent the cover from flipping up while being locked.
The method can be carried out, for example, with a cover having (i) an upper, substantially rigid shell portion, (ii) a lower, compliant undersurface secured to the shell portion, and (iii) means for releasably locking the shell portion to the collection tray. The undersurface of the cover can include, for example, a plurality of downwardly convex nodules (half-dome features), disposed in an array complementary to the well array. Further, the shell portion can include a plurality of landing sites along its upper surface configured to receive the lower end regions of the rods.
Still a further aspect of the invention provides a device for holding a plurality of rectangular, heat-sealable sheets.
In an exemplary embodiment, the device is comprised of a tray having a substantially rectangular bottom surface, four upwardly divergent sidewalls extending from the bottom surface, and an upper circumferential edge region defining a substantially rectangular open top. A plurality of ribs run along each sidewall, spanning most of the distance between the bottom surface and the upper circumferential edge region. Each of the ribs has a substantially linear surface that (i) faces an opposing sidewall and (ii) is substantially normal to a plane defined by the bottom surface of the tray.
According to one embodiment, a plurality of heat-sealable sheets, arranged in a vertical stack, is positioned in the tray such that peripheral side-edge regions of the sheets are disposed in contact with the substantially linear surface of each rib.
Another aspect of the present invention provides a method of sealing a rectangular, heat-sealable sheet over an array of wells held in a collection tray.
In one embodiment, the method includes the steps of (i) picking up a clear heat-sealable sheet; (ii) placing the sheet over open upper ends of the wells; and (iii) pressing a conformable heated surface against the sheet, from a side opposite the collection tray, with sufficient pressure such that the sheet is heat-sealed to the collection tray over the open upper ends of the wells. Further according to this embodiment, the conformable heated surface is pressed against the sheet using a plurality of spaced-apart elongated rods, disposed substantially normal to an upper surface of the collection plate. The rods can depend from a support structure positioned above the collection plate.
These and other features and advantages of the present invention will become clear from the following description.